Synthesis Of Doped Cobalt Disulfide And Its Performance For Electrocatalytic Hydrogen Evolution And Nitrogen Reduction

Rapid development of human society has caused serious energy crisis and environmental problems.The search for efficient and clean renewable energy is urgent.Electrocatalytic reaction,one of the core steps in the field of energy storage and conversion（such as water splitting,fuel cells,nitrogen reduction,etc.）,shows important significance for solving energy and environmental problems.In this process,efficient electrocatalytic materials are essential.Although noble metals such as Pd and Pt have good electrocatalytic activity,their large-scale use is limited by the poor abundance and high cost.To improve the reactivity and selectivity of the electrocatalytic process,designing and preparing efficient and stable non-precious metal-based electrocatalysts have become the key to the development of energy electrocatalytic technology.As a typical ransition metal sulfide,cobalt disulfide（CoS₂）has the advantages of low cost,high conductivity,and good stability,which makes it a potential electrocatalyst for electrocatalytic applications.However,the exposed active sites and poor catalytic activity of CoS₂ are difficult to meet the practical demands,seriously limiting their electrocatalytic application.To overcome above shortcomings,this dissertation proposes two kinds of doped CoS₂ nanoarray on flexible conductive carbon clothes（CC）,which were synthesized via a hydrothermal and high-temperature vulcanization process.The electrocatalytic pefromance of electrocatalysts was tailed by microstructure regulation and electronic modification strategies.On one hand,nanostructured nanoarrays have been constructed onto carbon cloth,and the structure is orderly,so that the catalytic sites of the catalyst are highly dispersed.The exposed area and active sites of the electrode are increased.On the other hand,the electronic regulation changes the intrinsic electronic structure of the catalyst,reduces the catalytic reaction energy barrier,accelerates the electron transport rate,and improves the intrinsic catalytic activity of the catalyst.In this dissertation,high-performance N-CoS₂/CC and Fe-CoS₂/CC electrocatalyts were explored via the strategies of microstructure regulation and electronic modification in combination with density functional theory（DFT）calculations.The specific research is as follows:1. Hydrothermal and high-temperature vulcanization methods are used to deposit N-CoS₂ nanosheets（N-CoS₂ NSs）on carbon cloth through hydrothermal and post-vulcanization processes.The effects of microstructure,hydrophilicity,and internal electronic structure on the electrocatalytic hydrogen production（HER）of this material were analyzed.The results show that regulating the electronic structure of the electrocatalyst by heteroatom doping is an effective way to improve the intrinsic catalytic performance of electrode materials.Due to the coupling between Co 3d and N 2p orbits,N-CoS₂ exhibits a narrower band width than CoS₂,which is beneficial to the electron transport inside the material.At the same time,due to the difference in electronegativity,XPS and DFT calculations revealed that the N atom deprived the electron from the Co site through the donor/acceptor interaction as the acceptor of the electron,and achieved the modulation of the electronic structure of the Co site at the reactive site.The hydrogen Gibbs adsorption energy at the Co site was reduced（0.4 e V→0.28 e V）.In addition,the microstructure regulation caused N-CoS2 NSs to expose many reactive sites.At the same time,N-CoS₂ NSs showed good air-repellency,promoted the escape of hydrogen,and avoided the hydrogen bubbles from affecting the catalyst surface Coating thereby blocks the progress of the reaction.N-CoS₂/CC exhibited enhanced electrocatalytic hydrogen evolution activity relative to CoS2.The Tafel slope decreased from 77.6 m V dec^-1 to 60.1 m V dec^-1,and the overpotential was only 112 m V at a current density of 10 m A cm^-2.2. The electronic structure of heteroatom-doped materials is an effective way to improve the intrinsic catalytic performance of materials.But different types of heteroatom dopant may induce remarkable different electrocatalytic performance.To solve this problem,Fe-CoS₂ NNs（Fe-CoS₂ NNs）growing vertically on carbon cloth were prepared by hydrothermal method and post-sulfidation method.The cationic Fe dopant,which shows far different chemical properties from N atoms,was chosed as the dopant.The results showed that the introduction of Fe slightly inhibited the activity of HER,but improved the electrocatalytic nitrogen fixation（NRR）activity,highlight the selectivity of heteroatom doping.The intrinsic reason for the enhanced NRR performance of Fe-CoS₂ was investigated.Fe-CoS₂ NNs are composed of interconnected Fe-CoS₂ nanoparticles,which have many mesopores and high electrochemical surface area(S_ECSA).Compared with the original CoS₂,Co 2p in Fe-CoS₂ shows a positive XPS binding energy shift,indicating that there is an electron transfer from Fe dopant to Co atoms.Differential charge density and Bader charge analysis confirmed the electron transfer of between Fe and Co.Further DFT simulations show that the NRR processes on Fe-CoS₂ and CoS₂ have the same rate-determining step（RDS）,that is,*NN→*NNH,regardless of whether the remote or alternative approach is considered.The Gibbs free energy barrier of RDS decreased from 1.41e V to 1.31 e V after Fe was incorporated into the CoS₂ lattice.The Fe-CoS₂/CC electrode shows good NH₃ production(6.31μg h^-1 cm^-2)and Faraday efficiency（3.39%）at-0.1 V vs.RHE,which is better than CoS₂ and most other metal sulfides.This enhanced activity of Fe-CoS₂ can be attributed to the optimization of Fe-doped electronic states and porous nanoneedles with large S_ECSA.Our work can provide reference for the effective utilization of other metal sulfide electrocatalysts.